Choosing a Well-Insulated Window
Windows that have good insulating values will make your home more comfortable,
particularly in winter. This is partly because they allow less heat to pass through, but
there's another reason - the inside surface of a better insulated window will be warmer.
When you stand or sit by it, your body won't lose as much heat to the window as it would
to a colder surface. Sometimes the draftiness that people feel from windows isn't due to
air movement at all, but rather to the fact that we radiate body heat to the cold window
This vivid example demonstrates the effectiveness of well-insulated windows
against condensation problems. The condensation visible on the single-paned
right-hand window is a result of the warm, humid, inside air coming in contact with the
cold glass and frame. The inside of the well-insulated window to the left stays
warmer, so condensation is less likely.
In addition to improving comfort, windows with high insulating values
are less likely to have problems with condensation. Condensation occurs when warm, moist
indoor air comes in contact with a cold surface, such as a poorly insulated window. Better
insulating values are most valuable in cold climates. The bigger the difference in
temperature between outside and inside, the faster heat will move through the window.
You'll notice the difference if you replace an old window with a better-insulating one.
For instance, when it is 0°F outside, the inside surface temperature of a double-pane
glass window is about 44°F, but for a high-performance window it jumps to about 56°F.
High-performance windows will help in the summer as well, particularly if you are trying
to cool the house to 78°F as the outside temperature climbs toward 100°F.
You can tell how much heat a window allows through by its U-factor, which measures thermal
conductivity. A lower U-factor means a better-insulating window. The more common term
R-value refers to the resistance of the window to heat conduction, and it is the inverse
of the U-factor (that is, R-value = 1/U-factor). Better windows have high R-values and low
U-factors (see Table 1).
Since the different parts of a window all have different U-factors, you should look at
the U-factor for the whole window. The frame and the edge of the glass usually have higher
U-factors than the center of the glass. If they don't specify-and they often do
not-manufacturers or dealers may refer to a window's center-of-glass U-factor, which is
almost always lower than the U-factor for the window as a whole.
Fortunately, many new windows are labeled with an energy information sticker from the
National Fenestration Rating Council (NFRC). The U-factor on the NFRC label always refers
to the whole window. To make sure you are comparing apples to apples, ask for the NFRC
ratings even when there is no label on the window (see "Window Ratings and Labels").
Also, be sure to use the same size windows for comparison, as the ratio of glass to
framing affects the result.
Whole Window U-factors of Sample Windows
frame w/o thermal break
frame with thermal break
Glass, ½" air space
glass, low-e, (E*=0.2), ½" air space
glass, low-e, (E*=0.1), ½" air space
glass, low-e, (E*=0.2), ½" space with argon
glass, low-e, on two panes, ½" paces with argon
glass, low-e (E=.01) on two panes, ¼" spaces with krypton
|*E is the
emittance of the low-e coated surface.
ASHRAE Handbook: Fundamentals, (Atlanta, GA:American Society of Heating, Refrigerating,
and Air-Conditioning Engineers, Incorporated, 1993).
Note: These are example of whole
window U-factors of 3 ft x 5 ft windows. U-factors vary somewhat with window size. Ask the
dealer for the specific values for the window you are looking at.
The first step to improving a window is usually to add a second pane of glass. This traps
a layer of still air, a good insulator, between the panes. Double-pane windows insulate
about twice as well as single-pane windows, so only half as much heat passes through the
The space between the two panes can also be filled with argon or, less often, krypton gas,
which insulate better than air. Krypton is somewhat more effective in windows with less
space between the panes (1/4 inch to 3/8 inch), so it is often used in windows with
multiple air spaces (such as triple-pane windows) to keep the thickness down. Windows
filled with air or argon work best when the space is about 1/2 inch. Windows with krypton
are usually more expensive, both because krypton itself is expensive and because the
designs tend to be upper scale. Argon is nearly as effective and does not add much to the
cost of a double-pane window.
features in this Superglass window include two low-e coatings, gas fills, and an
Wouldn't it be nice in winter if you could let in heat from
the sun, but keep the home's warmth inside? This is essentially what a low-emittance
(low-e) coating does. A clear microscopic metal oxide layer installed on a surface of one
of the panes of glass allows short-wavelength sunlight to pass through it, but reflects
long-wavelength infrared radiation. The heat energy from the home (and people) that
radiates toward windows is long-wavelength radiation, which is reflected back into the
Improved Frames and Spacers
Window frames are made from aluminum, wood, vinyl (polyvinyl chloride), or fiberglass.
There are also composites of two materials (for instance, vinyl and wood) mixed together
and formed or extruded like plastic. To achieve a certain look, manufacturers also offer
vinyl or fiberglass frames with a thin veneer of wood on the inside (wood-clad vinyl).
Others offer wood frames with a cladding of vinyl or aluminum on the outside for increased
durability. The frame can account for about 15% of the energy loss through a window.
Aluminum frames have high U-factors, unless they include a thermal break-a strip of
urethane that interrupts the transfer of heat through the metal.
Wood, vinyl, and fiberglass are much better insulators than standard aluminum frames
(without the thermal break). Of these, fiberglass performs slightly better than the rest
and is also the most durable. You'll find that vinyl and wood frames generally have
similar U-factors. Some very expensive vinyl frames are filled with urethane foam
In double- and triple-pane windows, the panes of glass are separated by spacers. The
spacers are traditionally made of aluminum, even in wood, vinyl, or fiberglass frames,
creating greater conductivity around the window edges. This makes the windows colder at
the edges in winter, and water vapor may condense there as it hits the cold surface. New
warm-edge spacers are made from better-insulating materials, and are recommended for cold
climates. The biggest advantage to warm edge spacers is that they reduce condensation
around the edge of the window.
Many people put up storm windows in the winter, and they do help. But storm windows are
typically fairly leaky. If you are deciding whether to buy storm windows or replace your
existing ones, you're probably better off putting your money into new double-pane windows.